This invention relates to hydraulic lawn mowers and more particularly to the control of hydraulically driven lawn mowers.
In one class of hydraulic lawn mower, a right hydraulic pump drives a right hydraulic motor to rotate a right wheel and a left hydraulic pump drives a left hydraulic motor to drive a left wheel. The mower is driven in a forward direction by driving both hydraulic motors at the same speed. The pumping speed of the hydraulic pumps is controlled by the operator through one or more levers, such as a ground speed lever.
This class of hydraulically driven lawn mowers includes a mechanism for avoiding unintended turning of the lawn mower when the forward speed is changed. The tendency to turn when the forward speed is changed occurs because the right and left hydraulic pumps, hydraulic motors and linkages connecting them to each other and to the ground speed lever have different speed characteristics. These different speed characteristics result in a different change in the speed of the right and left wheels with respect to each other when the ground speed lever is moved to change the forward speed of the mower.
Because a change in the ground speed lever affects the speeds of the right and left wheels differently under some circumstances, a change of the ground speed lever controlling the speeds of both the right and the left wheels results in a greater increase in speed of one wheel than in the other wheel. In some such hydraulically controlled lawn mowers, the same speed control lever that is used to change forward motion can turn the lawn mower. In this class of lawn mower, the speed control lever: (1) simultaneously changes the speed of the wheels in the forward direction or in the reverse direction by substantially the same amount when the lever is moved along a straight path in a longitudinal plane that corresponds to the forward direction of the mower; and (2) selectively changes the speed of the two motors with respect to each other by movement of the speed control lever at an angle to the longitudinal plane. Thus, the operator may control unintended turning from the speed control lever.
In one prior art apparatus for correcting the unintended change in direction of the lawn mower in the class of lawn mowers having a single speed control lever that changes the speed of both wheels by substantially the same amount when moved in a longitudinal plane and changes the speed of the right and left wheels with respect to each other when moved in a direction transverse to the longitudinal direction, the tendency to unintentionally turn is corrected from the single speed control lever but the speed control lever can turn the lawn mower by large amounts and is used to steer the lawn mower. In other prior art lawn mowers, correction against turning is made only by separate speed control levers which individually control the oil pressure from the right pump or the left pump.
The prior art hydraulic lawn mowers have several disadvantages, such as for example: (1) it is difficult to avoid unintended turning; and (2) there is an increased tendency to turn by an excessive amount, thus creating under some circumstances, a hazard.